This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-123867 filed on Apr. 25, 2002.
1. Field of the Invention
The present invention relates to an exhaust gas purification system that has a particulate filter for trapping particulate matters included in exhaust gas discharged from an internal combustion engine. Specifically, the present invention relates to an exhaust gas purification system capable of precisely measuring a quantity of the trapped particulate matters.
2. Description of Related Art
Various devices for reducing particulate matters discharged from diesel engines are proposed as measures for protecting the environment. A typical example of such devices is a diesel particulate filter (DPF) disposed in an exhaust pipe. The DPF adsorbs and traps the particulate matters included in exhaust gas when the exhaust gas passes through porous filter walls of the DPF. The surface of the filer wall may be coated with an exhaust gas purification catalyst or an oxidization catalyst. If a quantity of the trapped particulate matters increases, pressure loss in the DPF increases and deficiency such as lowering of engine output may occur. Therefore, the DPF is regenerated regularly by burning and eliminating the trapped particulate matters. Thus, the DPF can be used continuously.
As shown in Japanese Patent Unexamined Publication No. H7-310524, a time interval of the regeneration of the DPF is determined based on whether the quantity of the trapped particulate matters (PM trapped quantity) is greater than a predetermined value or not. The PM trapped quantity is calculated from a pressure difference between the upstream of the DPF and the downstream of the DPF. The DPF is regenerated by heating the DPF up to the temperature at which the particulate matters can be burned. The DPF is heated by heating means such as a burner or a heater, or by high-temperature exhaust gas generated by decreasing an intake air quantity or performing post injection. In order to burn the particulate matters stably, it is important to suitably set the time interval of the regeneration by precisely measuring the PM trapped quantity.
The particulate matters mainly consist of a soluble organic fraction (SOF) and a soot fraction. The SOF fraction includes oil, unburned fuel and the like. Specifically, the SOF has a significant effect on the pressure difference between the upstream and the downstream of the DPF because the SOF exists in various states such as a gaseous state or a liquid state, depending on a state of combustion in a cylinder. It is because the SOF adheres to the soot particulates in various states depending on the state of the SOF, and a clogging degree of the pores of the DPF varies in accordance with the variation in the adhering state of the SOF. In addition, a ratio between the SOF and the soot fraction varies, depending on operating state of the engine. Therefore, the PM trapped quantity varies even when the pressure difference between the upstream and the downstream of the DPF is the same. Thus, it is difficult to calculate the precise PM trapped quantity only from the pressure difference between the upstream and the downstream of the DPF.
A device disclosed in Japanese Patent Unexamined Publication No. H7-310524 has operating state detecting means and correcting means for correcting the PM trapped quantity based on the ratio between the SOF and the soot fraction, in order to solve the deficiency caused by the variation in the composition of the particulate matters. The ratio between the SOF and the soot fraction is estimated from the operating state of the engine. However, even in this scheme, the effect of the variation in the state of the SOF cannot be eliminated fully. If the PM trapped quantity is calculated to be smaller than an actual quantity, excessive particulate matters may be trapped and excessive heat may be generated by a rapid burning of the particulate matters when the DPF is regenerated. A graph in FIG. 4 shows a relation between the PM trapped quantity M and the maximum temperature TMAX in the DPF in a case in which the rapid burning of the particulate matters occurs. As shown in FIG. 4, the maximum temperature TMAX in the DPF increases from normal temperature T0 as the PM trapped quantity M increases. Moreover, an increasing degree of the maximum temperature also increases as the PM trapped quantity M increases. Accordingly, a possibility of damage to the DPF caused by excessive heat generation may increase as the PM trapped quantity M increases. On the other hand, if the PM trapped quantity is calculated to be larger than the actual quantity, frequency of the regeneration of the DPF may increase. As a result, fuel consumption of the engine may increase, or a quantity of the fuel provided to the burner or power consumption by the heater may increase. Therefore, stable regeneration of the DPF at suitable timing, which is achieved by a precise measurement of the quantity of the trapped particulate matters, is required.
It is therefore an object of the present invention to precisely measure a quantity of trapped particulate matters independently of operating states of an engine and to perform regeneration of a diesel particulate filter at suitable timing. Thus, the regeneration of the diesel particulate filter is performed safely and surely.
According to an aspect of the present invention, an exhaust gas purification system of an internal combustion engine has a particulate filter and regeneration controlling means. The particulate filter is disposed in an exhaust pipe of the engine and traps particulate matters included in exhaust gas. The regeneration controlling means measures a quantity of the trapped particulate matters and performs regeneration of the particulate filter based on the measured quantity of the trapped particulate matters. The regeneration controlling means heats the particulate filter at least to a predetermined temperature at which a soluble organic fraction (SOF) included in the particulate matters can be eliminated. After that, the regeneration controlling means measures the quantity of the trapped particulate matters based on pressure loss generated in the particulate filter.
In order to precisely calculate the quantity of the trapped particulate matters, the particulate filter is maintained at the predetermined temperature or above and the SOF included in the particulate matters is eliminated. Thus, the composition of the particulate matters depositing on the particulate filter is changed so that the particulate matters consist of a soot fraction alone. Then, the quantity of the trapped particulate matters is calculated based on a pressure difference between an upstream and a downstream of the particulate filter and the like. Thus, the quantity of the trapped particulate matters is calculated precisely independently of the composition of the particulate matters or a state of the SOF. Therefore, excessive increase in the temperature of the particulate filter during the regeneration and an increase in fuel consumption are prevented. Thus, timing of the regeneration of the particulate filter is set suitably, and the particulate filter is regenerated safely.